Organics and nutrient recovery from anaerobic digester residues

ABSTRACT

Sludge from an anaerobic digester is treated to recover one or more of fibers, or solids or liquids with a high nutrient content. The solids or liquids can be used as a fertilizer. The fibers can be used in a plant growing medium. Solids are separated from liquids in the sludge and dried. The solids may be dried to produce a flake or pellet. Ammonia in the liquids is recovered and used to produce a concentrated acidic ammonium salt solution. This solution may be mixed with the solids to produce a nitrogen enhanced solid. The fibers and solids or liquids can also be used in combination to produce an enhanced plant growing medium. A device and process for removing ammonia from a liquid can be used in the system or separately.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is division of U.S. patent application Ser. No.13/982,585, filed Oct. 15, 2013, which is a national phase entry ofPCT/CA2012/000144, filed on Feb. 17, 2012, which claims the benefit ofU.S. Provisional Application No. 61/443,905 filed on Feb. 17, 2011 andU.S. Provisional Application No. 61/578,703 filed on Dec. 21, 2011. U.S.patent application Ser. No. 13/982,585, PCT Application No.PCT/CA2012/000144, U.S. Provisional Application Nos. 61/443,905 and61/578,703 are incorporated by reference.

FIELD

This specification relates to the recovery of organics and nutrientsfrom waste, to anaerobic digestion alone or in combination with awastewater treatment plant, to products such as fertilizer or compostmade from anaerobic digester residue device, and to a method forremoving ammonia from water, such as sludge dewatering centrate.

BACKGROUND

The following discussion is not an admission that anything discussedbelow is common general knowledge or citable as prior art.

Various organic waste products contain nutrients that make the wastepotentially valuable as fertilizer. For example, some animal manures andorganic sludges or slurries could be applied directly to land. However,due for example to the large quantities of material involved relative tothe nutrient content, and potential problems with odors, this practiceis limited to selected appropriate operations located near the source ofthe waste. The manure, sludge or slurry might be treated to remove largefibers, physically dewatered, partially dried thermally, extruded into asolid fertilizer product and then further thermally dried. However sucha product would not be stable and would tend to decompose or attractmold during storage because of its high biodegradable organic mattercontent. Alternatively, manures, sludges or slurries could be digestedin an anaerobic digester to produce a biogas. The digested sludge couldthen be applied to the land as a fertilizer. While the biogas producedis useful as a fuel, use of the digester sludge as a fertilizer is stilllimited to selected appropriate operations near the source of the waste.

In an activated sludge wastewater treatment plant, ammonia is removedfrom the wastewater at least in jurisdictions with relevant dischargeregulations. In these plants, waste activated sludge may be sent to ananaerobic digester. Sludge from the digester, comprising digestate, istypically de-watered before it is disposed or treated further. Theliquid stream from the de-watering device, which may be called rejectwater, centrate or filtrate, is often returned to the main activatedsludge process. This centrate contains ammonia, and there have been someattempts to remove ammonia from the centrate before it is sent back tothe main process. A paper by Tim Constantine, presented at the 2006WEFTEC conference and entitled “North American Experience with CentrateTreatment Technologies for Ammonia and Nitrogen Removal”, provides asummary of ammonia removal technologies that have been used in NorthAmerican facilities.

US Patent Application Publication Number 2007/0297953 to Kemp et al.describes a system in which ammonia is removed from water in a vacuumassisted flash stripping tower. The water is treated before stripping toremove solids removal tank and multivalent cations and increase its pH.

U.S. Pat. No. 7,416,644 to Bonde describes a fermenter with a sidestream ammonia stripping step. Ammonia is stripped from fermentedbiomass in a shunt. Effluent from the fermenter passes through the shuntwhile water vapor is injected into the shunt.

INTRODUCTION

The following paragraphs are intended to introduce the reader to themore detailed description to follow, and not to limit or define anyclaimed invention.

This specification describes, among other things, a fertilizer product,a method of making a fertilizer product, a method of treating anaerobicdigester sludge and a waste treatment process including anaerobicdigestion. In brief, sludge from an anaerobic digester is treated toproduce a generally dry nitrogen rich fertilizer product, which may becalled a pellet or a granule herein.

In a treatment plant and process described in further detail below,solids are separated from liquids in the sludge and dewatered, dried orboth. The liquids in the sludge contain aqueous ammonia that is releasedin one or more gasses or liquids produced during dewatering or drying.These liquids or gases are collected and then treated in an ammoniarecovery system to produce a concentrated acidified ammonium saltsolution. This solution, relative to the liquid in the digester sludge,has a higher concentration of ammonia, reduced alkalinity and reducedpH. The acidic ammonium solution is reintroduced to the dried solids toproduce a moist pellet. The moist pellets are then dried at ambient tomoderate temperatures, for example by a flow of warm air over thepellets. After drying the pellets, ammonia from the recovered liquidremains with the pellets as an ammonium salt.

Anaerobic digester sludge is more stable than the undigested feedstockbecause it has a reduced concentration of biodegradable solids.Nevertheless, anaerobic digester sludge contains carbon and nitrogen,among other nutrients, in mineralized and organic forms that are usefulas fertilizer. However, the nitrogen exists primarily in aqueous formsof ammonia. A typical digester sludge dewatering process would thereforelose much of the ammonia with removed water. Further, the liquid in thedigested sludge also has a high pH and is heavily buffered withalkalinity. Heating the de watered sludge cake under typical sludgedrying temperatures, given its high pH, would convert the ammoniaremaining in the liquid in the cake primarily into ammonia gas,resulting in more loss of ammonia along with the evaporated water.

However, in a process and apparatus described herein, one or more of theliquids, vapors or gases produced by dewatering or drying the sludge, orboth, are collected and processed in an ammonia recovery system. In therecovery system, water or vapor with an increased concentration ofammonia and reduced alkalinity is created and mixed with an acid. Wthreduced volume and alkalinity (relative to the water in the digestersludge), a reasonable amount of acid is able to produce an ammoniacontaining liquid with low pH. Further, the volume of liquid carryingthe recovered ammonia is reduced to the point where it is feasible toreintroduce the concentrated ammonia liquid into the dried solids in apellet making process. The produced pellets are moist and have a highsurface area per unit volume, allowing drying by way of a flow of air atmoderate temperature to produce a pellet dry enough, considering thestabilized nature of the solids, for storage and transport. With themoisture in the pellet at a low pH and drying at moderate temperatures,ammonia ions in the moist pellet tend to precipitate as salts thatremain with the pellets rather than forming ammonia gas. In this way,the apparatus and process described herein produce a pellet with highernitrogen content than would be achieved merely by dewatering, drying andpelletizing the anaerobic digester sludge.

This disclosure also describes a system and process to recover fibers,or solids or liquids with a high nutrient content, or both, fromanaerobic digester residues. The fibers can be used in a plant growingmedium. The solids, for example in a granule or flake form, or liquidscan be used as a fertilizer. The fibers and solids or liquids can alsobe used in combination to produce a plant growing medium.

A device and process for removing ammonia from a liquid are describedherein. The ammonia flows through a series of sequential stages.Bubbles, for example of air, are provided in the liquid in the stages.At the same time, air flows across the surface of the liquid in thestages. The flow rate of the surface flow is greater than the flow rateof the bubbles.

The device and process for removing ammonia can be used in the systemand process to recover fibers, or solids or liquids with a high nutrientcontent, or both, from anaerobic digester residues. Alternatively, thedevice and process for removing ammonia can be used in otherapplications, for example removing ammonia from municipal wastewaterplant digester centrate or other waste streams with ammonia. The deviceand process for removing ammonia can be used in combination with acommercial acid or ammonia scrubber.

Elements of the various systems and process described herein may becombined. For example, solids, liquids or vapors produced in a systemand process described herein for making flakes can be used to make apelletized fertilizer as described herein.

FIGURES

FIG. 1 is a process flow diagram for a plant for treating sludge toproduce a fertilizer pellet, coupled with an anaerobic digester toproduce the sludge from a waste stream.

FIG. 2 is a schematic process flow diagram of a nutrients recoverysystem including an ammonia removal system.

DETAILED DESCRIPTION

FIG. 1 shows a plant for producing a solid fertilizer product, pelletsQ, from sludge, particularly anaerobic digester residue or digestate B.This plant is coupled with an anaerobic digester 1, which produces thedigestate B from a waste stream or feed stock A. Examples of suitabledigester feedstock A that results in high nutrient content digestate Binclude animal manure, post consumer food waste, pre consumer foodprocessing waste, biofuels processing by products, agricultural waste,and municipal wastewater sludge, among others. The origin and nature ofthe feed stock A, and possibly the type of chemicals used in thefertilizer production process to be described below, may allow thefertilizer product to be labeled as “organic” or by another related termin accordance with applicable regulations. The product may be called apellet or a granule. Either of these words may be used herein to denotea substantially dry product in the form of many small (for example 1 mmto 50 mm in the largest dimension) pieces, but without intending tolimit the product to any particular size or shape of product.

Solid fertilizer products typically have a higher value than raw wastesor liquid fertilizer products because a solid product facilitatestransporting, storing and using the fertilizer with less cost and withreduced nuisance, particularly odors. In general, it is desirable for asolid fertilizer product to have a substantial concentration ofnutrients, including nitrogen. The product should also have a lowconcentration of pathogens and be organically stable such that it doesnot decompose and grow mold readily in storage. It is also advantageousfor the product to be sufficiently hard, uniformly sized and flowablethrough machinery so as to allow the product to be stored, transportedand broadcast with conventional dry fertilizer application equipmentused in agriculture and horticulture. The size and shape of a pellet Qcan be made to satisfy the physical requirements described above. Thesteps involved in processing the digestate B, to be described in moredetail below, are intended to avoid the loss of nutrients, particularlynitrogen, that might otherwise occur if digestate B were more simplydewatered, dried and pelletized.

For comparison, some animal manures and organic sludges and slurriescould be dewatered without anaerobic digestion and then thermally driedand extruded, after removing large fibers, to produce a pellet. Such aproduct would not be stable and instead would be prone to decompose andgrow mold during storage. The product would also have a significantlyreduced nitrogen content compared to the feedstock since most of thesoluble nitrogen present in the raw waste would be lost duringdewatering and drying and would not be incorporated into the pellets.

Alternatively, and for further comparison, a manure, sludge or slurrycould be first digested in an anaerobic digester and then mechanicallydewatered, thermally dried and pelletized. The use of an anaerobicdigester allows a biogas to be created and collected that can be used asa fuel for power or heat generation, or both. Anaerobic digestion alsoreduces greenhouse gas emissions relative to allowing organic waste todecompose to methane in the soil. The resulting pelletized sludge wouldalso be more stable than the dried pelletized raw waste discussed abovesince many of the organic compounds in the raw waste are mineralized inthe anaerobic digestion process, and in particular the concentration ofcarbon in biodegradable forms is greatly reduced. For these reasons,digesting the waste stream would be an improvement over simplypelletizing organic waste. However, the nitrogen content in the pelletswould still be low since most of the organic nitrogen would be convertedto ammonia that would exist primarily in the liquid fraction of thesludge. This ammonia would again be lost, for reasons that will beexplained in more detail below, first with the liquid fraction removedduring dewatering and then as ammonia gas during the thermal dryingprocess.

The plant of FIG. 1 uses digester residue to produce fertilizer pelletsbut differs from the alternative described above in that additionalsteps are provided to retain nitrogen contained in the liquid fractionof the digestate B. The result is a stable fertilizer pellet, but withincreased nitrogen compared to simply pelletizing the digestate solids.The fertilizer producing apparatus could be located separately from theanaerobic digester. However, when the fertilizer producing apparatus isco located with the anaerobic digester 1 as in FIG. 1, the need to movedigestate B or intermediate products is reduced, the biogas or wasteheat from power generation can be used in the fertilizer manufacturingprocess, and waste liquid streams may be advantageously returned toanaerobic digester 1.

As mentioned above, the nutrients in organic waste are partiallymineralized, or converted into inorganic forms, in the anaerobicdigestion process. Organic waste streams typically contain a combinationof volatile and non volatile, or inert, solids. Volatile solids maycomprise 70 to 90% of the solids fraction in typical waste streamsamenable to anaerobic digestion. Depending on the nature of the volatilesolids only a fraction, usually ranging from 40 to 80%, is anaerobicallydegradable by bacteria in digesters and is converted into methane,carbon dioxide and water. The solids remaining in the digestate stillcontain some carbon, and the loss in carbon has been compensated for bythe production of biogas. The digestate also has generally unchangedamounts of other nutrients such as nitrogen, phosphorous and potassium.These nutrients tend to be mineralized during digestion and theinorganic forms of the nutrients may be more useful to plants than theorganic forms. Therefore, in addition to being more stable due to thereduction in organic carbon, applying digestate to the land may providemore nutrient value to crops compared with the raw waste. However, someof the mineralized nutrients are aqueous or suspended. Since the goal isto produce a dry product, solids need to be separated from liquid in thedigestate, This is typically done by mechanical separation (de watering)processes followed by thermal drying, meaning drying at a significanttemperature for example 100 degrees C. or more. The effect of theseprocesses on the nutrients phosphorous, potassium and nitrogen isdiscussed further below.

Phosphorous, in manure for example, is present mostly as organicphosphorous associated with particulate organic matter and dissolvedunreactive phosphorous comprising organic phosphorous andpolyphosphates. A minor proportion is dissolved reactive phosphorus ororthophosphate. Substantially all of the phosphorous present in themanure will still be present in the digester sludge. During anaerobicdigestion, organic phosphorous contained in volatile solids and biomasssolubilizes and adds to the soluble organic phosphorous present in thewaste. The soluble organic phosphorous mineralizes and becomes adsorbedto particulate bound solids. Because phosphorous does not easily formgasses, it tends to stay in the manure or other substrates throughdigestion. Solids separation (de watering) operations performed on thesludge may partition up to 70% of phosphorous in a cake portion of thesludge, particularly if the solids separation process is augmented withcoagulants. If only dewatering flocculants are used, then about 50% ofthe phosphorous may remain in the cake after de watering. Thephosphorous contained in the liquid fraction of the cake will besubstantially retained as solids when water evaporates as the cake isthermally dried.

Potassium is not highly reactive and is mostly present in a soluble formin manures and other organic slurries and sludge. Potassium remainsessentially unchanged during digestion and it does not become a gas ondrying. During sludge de watering, some potassium will remain in theliquid fraction removed and some will remain in liquids that are part ofthe solids fraction, or cake. When the cake is dried, potassiumcontained in the liquid portion of the cake will remain as a solid whenwater evaporates.

Nitrogen may be present in the feedstock as urea, amino acids, protein,and various other forms of particulate and soluble organic nitrogen.During anaerobic digestion, these organic forms undergo mineralizationand are converted primarily into dissolved (aqueous) ammonia andammonium. Passing through a digester has little effect on the totalnitrogen content of the waste. A negligible amount of nitrogen may beemitted as NH3 (unionized ammonia gas), but the majority will be foundas NH4 (ionized ammonia or ammonium) or ammonia gas in solution in theliquid fraction of the digester sludge, and a minor proportion asorganic nitrogen in undigested volatile solids. The ammonium content ofdigester sludge is usually higher than that of the raw waste. Therelative presence of ammonia (NH3 gas) and ammonium (NH4+ ion insolution) in the liquid of the digestate is a function of pH andtemperature. A larger fraction is present as unionized ammonia (NH3 gas)with increased temperature and with increased pH. In the mesophilic andthermophilic range of digesters, operating at 35 to 55 degrees Celsiusand at a pH of between 7.5 and 8.2, most of the reduced nitrogen existsas ammonium ions. Total ammonia concentrations are typically not allowedto exceed about 5000 ppm in mesophilic reactors and about 3000 ppm inthermopohilic reactors since the unionized ammonia fraction is toxic tomethanogenic organisms. Therefore digesters for manures with high solidsand high nitrogen content, such as digesters for poultry manure, aretypically diluted.

During mechanical de watering of digestate, most of the nitrogen will beremoved in the liquid fraction as soluble ammonium. The cake portionwill only contain the ammonium dissolved in the liquid portion of thecake and organic nitrogen contained in undigested volatile solids.However, particularly in digestate containing a high nitrogenconcentration that would be useful to produce fertilizer, the pH may beas high as 8.2 and the alkalinity can be as high as 8000 to 20000 mg/Las CaCO3. At this relatively high pH, when temperature increases duringthermal drying of the cake, most of the ammonium contained in the cakemoisture will shift to ammonia gas and will be driven off the cake alongwith the evaporated moisture. This further reduces the nitrogen contentin the dried solids, and the nitrogen that remains will be mostlyorganic N that is not readily available to crops. Attempting to retainsome of the nitrogen in the liquid by adding acid to the cake to reducethe pH would not likely be cost effective. The liquid in the cake iswell buffered by the alkalinity and would require very large amounts ofacid to be added to the cake to significantly reduce the pH.

In summary, for phosphorous and potassium, some but not necessarilymost, of the nutrients are removed with water during mechanicaldigestate dewatering but remaining nutrients remain in the cake afterthermal drying. In contrast, most of the nitrogen in digestate isremoved with water during mechanical de watering, and most of thenitrogen that remains is driven off as a gas during thermal drying.Accordingly, and because nitrogen is arguably the most importantnutrient, particular attention is paid to retaining nitrogen in theproduct fertilizer in the process that will be described below.

The process and apparatus of FIG. 1 produces a fertilizer pellet Q froma digestate B or, when coupled with an anaerobic digester 1, from awaste stream or feed stock A. The process recovers at least some, andpreferably most, of the mineralized nitrogen present in the digestate Bas an ammonium salt that is incorporated into the pellet Q. Thisincreases the nitrogen content of the pellet Q, relative to merelydewatering and drying the digestate B, and provides the nitrogen in afrom that is readily available to crops. Since the nitrogen in theammonium salt was originally present in the digestate B and the feedstock A, the pellet Q may qualify as a natural or organic fertilizerdepending on applicable regulations.

In FIG. 1, animal manure or other digester feedstock (A) enters ananaerobic digester (1). Digested sludge or digestate (B) goes to amechanical dewatering device (2), which can be for example a centrifuge,screw press, belt press, rotary press or any other mechanical dewateringdevice. Optionally, a flocculant or polymer (T) is added to helpflocculate digested solids and increase solids capture and cake dryness.The dewatering device produces a cake (C) with solids content ranging,for example, from 14 to 30% or more depending on the digestate and thetype of dewatering device used. The dewatering device (2) also producesa liquid stream (D) called centrate herein although filtrate or pressateor other words may be more appropriate depending on the dewateringdevice used. The cake (C) goes to an indirect thermal dryer (3) that mayuse, for example, biogas, natural gas or electricity as an energy source(V) to evaporate water from the cake. The dryer (3) can be, for example,a hollow screw type dryer with steam or hot oil circulation, a disc typedryer or a press type dryer, etc. Dry cake (K), though not absolutelydry, may be referred to as a solids fraction on the digestate (B).

An indirect enclosed dryer is used such that gasses (E) from the cakedoes not mix with combustion air or other gases, or with dust that thesolids may produce in the drying process. Gasses (E) emitted from thecake (C) in the dryer will include water vapor and ammonia gas thatevolves from liquid in the cake (C) as a result of a shift from ammonium(ion in solution) to ammonia (gas in solution) as the liquid is heatedin the dryer (3). The gases (E), comprising water vapor and ammonia, goto a condenser (10), for example an indirect condenser that uses openloop or re circulating cooling water (A, B). Here vapor becomescondensate (U) comprising liquid water and ammonia in solution. It isdesirable to maintain the condensate at a high temperature, for example90 degrees C. or more. The condensate (U) and centrate (D) may becombined in a storage tank, which may be a separate tank or part of anammonia recovery system (4). The relative high temperature of thecondensate (U) increases the temperature of the combined liquid (D+U),for example to about 40 to 45 degrees Celcius, which is useful for asubsequent ammonia recovery step. The combined liquid (D+U) likelycontains some solids from the centrate (D), but may be referred to as aliquid fraction of the digestate (B).

The combined liquid (D+U) is fed to an ammonia recovery system (4). Theammonia recovery system (4) can include, for example, a steam stripper,a vacuum and heat based stripping system, or an air stripper. In theammonia recovery system, the combined liquid (D+U) is treated to releaseammonia gas, typically with water vapor. The gas/vapor mix is collectedand condensed as ammonium hydroxide (ammonia water) with an acid addedto produce an acidic ammonium salt solution. The gas/vapor mix may becondensed before the acid is added, or the ammonia gas and water vapormixture can be mixed without a distinct condensing step into an acidicsolution. Optionally, the ammonium salt solution may then be furtherconcentrated. Further optionally, additional acid may be added to theammonium salt solution to further reduce its pH.

Although any form of ammonia recovery system might be used, centrates(D) with high ammonia content, which are thus suited for makingfertilizer, also tend to have high alkalinities. For instance chickenmanure digester centrate with a concentration of about 5000 mg/L ammoniaN maintained by dilution in the digester (1), can have 16000 to 20000mg/L of alkalinity. In the digester (1), carbon dioxide in the biogasreacts with ammonium to form ammonium carbonate, a strong bufferingsystem. Such centrates (D) are well buffered and would require a largeamount of caustic to increase the pH. Therefore, some ammonia recoverysystems which would use a caustic to drive ammonia gas out of thecombined liquid (D+U) would not be as cost effective as other systemsdue to the high chemical cost. For example, one system produced byEnvimac Engineering GmbH relies on raising the pH to over 9.4 to drivethe ammonium in a liquid to ammonia gas, and then strips the ammonia gasusing from falling liquid using counter current air. A packed media bedis used to increase the surface area of a top sprayed liquid forimproved mass transfer with the air. The air stripping is followed by anacid scrubber step. However, as discussed above, a large amount ofcaustic would be required to raise the pH of the buffered combinedliquid (D+U).

Another option for recovering ammonia is using steam stripping. Ammoniaremoval systems using steam are also available, for example, fromEnvimac Engineering GmbH. These methods use less chemicals than airstripping methods, but require more energy than air stripping methods.Steam stripping methods may be particularly useful in plants where thecombined liquid (D+U) flow rate is low relative to waste energyavailable to produce steam. Waste energy may include, for example,energy available from heat recovery steam generators using the exhaustgases from internal combustion gas engines operated with biogas indigestion plants, or low pressure steam boilers operating with biogas orother fuel that would not otherwise be used. A first steam decarbonationstep may be used to drive carbon dioxide from the combined liquid (D+U)and reduce its buffering capacity. Some caustic is then added to raisethe pH of the liquid, and an ammonia steam stripping step is appliedwherein the liquid drops through a column of rising steam. Due to theheat of the stream, the ammonia can be driven out of the combined liquid(D+U) at a lower pH and so less caustic is required. The ammonia gasforms ammonium hydroxide (ammonia water) with the steam. An acid canthen be added to the ammonium hydroxide to form a stable ammonium salt.

Another, and possibly preferred, option for an ammonia recovery system(4) uses a flash vacuum distillation systems in which ammonia gas isextracted from the combined liquid (D+U) using heat and vacuum to shiftthe ammonium to ammonia. The combined liquid (D+U) may be heated toabout 80 degrees C. and then sprayed as a mist into a column under avacuum, which causes the ammonia to be released from the liquid. Theammonia gas and some water vapor are collected in flow of air risingupwards through the column towards an inlet to the vacuum source. In theROAST system produced by ThermoEnergy Corporation of Worscester, Mass.,the vacuum source is a venturi nozzle through which water or an acidicsolution is recirculated. When an acidic solution is recirculated, thecollected gas/vapor mixture is drawn directly into the acidic solutionto produce an ammonium salt solution generally simultaneously withcondensation.

In an ammonia recovery system (4) requiring heat, heat of evaporationintroduced to the dryer (3) may be recovered in the condenser (10) ashot water (AB), and that heat may be sufficient to further increase thetemperature of the centrate and condensate mix (D+U) to about 50 degreesCelcius or more, possibly to 70 or 80 degrees Celcius. The salt producedin the ammonia recovery system may be, for example, ammonium sulfate,ammonium acetate or ammonium citrate, depending on the acid used.Ammonium sulfate in particular is accepted as a useful fertilizer.

The concentration of the ammonium salt solution initially produced inthe recovery system (4) may be such that using all of the ammonium saltsolution would introduce too much water to the mixer (6) and pelletizer(7) to be described below. Excess ammonium salt solution could be soldas a liquid fertilizer. Alternatively, the recovery system (4) mayinclude an ammonium salt solution concentration step. The ammonium saltsolution may be concentrated to, for example 35 to 45%, at whichconcentration all of the ammonium salt solution may be used in thepellets. The concentration can be done, for example by thermaldistillation, by using gas permeable membranes or by flash evaporation.By any of these methods, water vapor is produced and removed from thesalt solution. One suitable system is the CAST system, a modified flashevaporation system, produced by ThermoEnergy Corporation.

In summary, depending on the ammonia recovery system (4) used, ammoniais stripped from the combined liquid (D+U) by either increasingtemperature by introducing heat or steam (S), and/or increasing pHadding a strong base such as sodium hydroxide (VV) and/or reducingpartial pressure by introducing vacuum. For ammonia recovery systemsthat rely on increasing the temperature of the liquid it is desirable touse the heat of evaporation recovered in the condenser (10), which isthe heat of evaporation from the water removed in vapour (E) from wetcake in the dryer (3). In the ammonia recovery system (4), the ammoniumis converted to ammonia gas, released from the combined liquid (D+U) andcaptured, typically in solution with water or water vapor from one ormore of vapors from the combined liquid (D+U), stripping steam orrecirculating liquid producing a vaccumm. The ammonia gas released fromthe condensate/centrate stream (D+U) is reacted with an acid (F) to forma stable ammonium salt in solution. The acid added can be sulfuric,acetic, citric, or other. The resulting ammonium acid solution ispreferably further concentrated, for example to 35 to 45%, for exampleusing flash distillation with vacuum, heat, a gas permeable membrane, orcombinations of these to produce a concentrated ammonium salt solution(G). In the ammonium salt concentration process, excess acid ispreferably added to produce a salt solution with a low pH, for example 5or less, or 3.5 to 5.

After ammonia recovery, the remainder of the centrate/condensate stream(D+U) exits the recovery unit (4) as an effluent (H) with reducedammonia content. Ammonia removal ratios ranging from 40 to 90% arepossible. The effluent (H) with reduced ammonia can be discharged tosewer, further treated for discharge to the environment or recirculatedto the digester (1). The effluent (H) contains some phosphorus andpotassium that can thereby be reintroduced into the digester (1).Further, the effluent (H) can function as dilution water to reduce thesolids and ammonia content in the digester. As an example, in digesters(1) treating chicken manure, dilution water is required to reduce thesolids content of the manure and reduce the ammonia concentration in thedigestate to avoid the toxic effects of unionized ammonia on methaneforming bacteria (methanogens). Returning the low ammonia effluent (H)to the digester (1) is useful to reduce the amount of new fresh dilutionwater required. When using ammonia recovery systems that require heat,the effluent (H) may exit at a relatively high temperature (for example50 to 70 degrees C.). Using this as dilution water for the digester (1),if required, also contributes heat useful for operating the digester atpreferred temperatures (for example 35 to 55 degrees C.) when returnedfor dilution of the digester feed (A).

Returning to the dryer (3), a dry solids stream (K) leaves the dryer (3)with, typically, 90 to 98% solids content. The dried solids (K) can exitas a flake, granule, or an aggregate depending on the dryer technologyused. The dry solids (K) may be cooled in a discharge cooling conveyorbefore further processing. The dry solids (K) may pass through a crusher(5) to reduce the size of the granules, flakes or other forms of solidclumps coming out of the dryer (3). Crushed solids (L) go to a pin orother type of mill or mixer (6). In the mill (6), crushed solids and afirst portion of the ammonium salt solution (I) are mixed. The resultingconditioned pelletizer feed (M) may have a solids content of, forexample, 65 75% solids. The mill (6) homogenizes and conditions thematerial to introduce it to a granulator, for example a disc or panpelletizer (7). In the pelletizer (7) the remaining ammonium saltsolution (J), for example 10 30% of the ammonium salt solution (G), isintroduced using spraying nozzles. The added liquid sprayed on thegranules being formed by the rotation action of a pan pelletizer (7)acts as binder to help form granules. In an alternative processconfiguration, an extruder may be used instead of the disc granulator(7). The extruder uses a positive displacement pump or press to drivethe conditioned material from the mill (6) through an extrusion die.

A granulated or extruded moist (or “green”) pellet (N) is produced withapproximately 60% to 70% solids content as a result of the moistureadded with the ammonium salt solution (I+J). The ammonium salt becomespart of the moist pellet (N), increasing its nitrogen content andslightly reducing the pH of the moist pellet (N). The moist granule orpellet (N) goes to a dryer (8), for example a low temperature beltdryer, where air (AA) and heat (V) are introduced to dry the greenpellet (N). The dryer (8) may operate at temperatures below 90 degreesC., for example between 60 and 80 degrees Celsius. The low temperaturein conjunction with the low pH, particularly if the pH is furtherreduced with an acidic ammonium salt solution (G), minimizes the shiftof ammonium to ammonia gas on drying and so minimizes ammonia loss inthe dryer (8). As a result most of the ammonia introduced with theammonium salt solution (G) remains as solids in the dry pellet (0). Thepellets (0) may have a small nominal diameter, for example 1 to 4 mm, toprovide a large surface area to further enable efficient direct dryingwith heated air at low temperature. Moisture is removed and the greenpellets gain strength. Dry pellets (0) exit with 90 to 92% solidscontent.

Optionally, a sizing screen (9) classifies the dry pellets (0) within aspecified size range. The undersized and oversized pellets (P) may go tothe feed crusher (5) where they are crushed and blended with the solids(K) out of the dryer to feed the conditioning mill (6). The pelletsretained between the screens in a specified particle size range, forexample retained between 1 mm and 4 mm screens, exit the screenclassifier (9) as finished pellets (Q). A transport device, such as abucket elevator, may convey the finished pellets (Q) to a bagging unit(11) which may have, for example, a hopper, a filling head and a scale(11). The finished pellets (Q) may be bagged, for example, in 1 tonsuper sacks (R) for storage and distribution. Alternatively the finishedpellets (Q) can be bagged in smaller bags, for example 5 to 50 poundbags, for distribution to retail stores.

The air exiting the belt dryer (X) may contain dust and may be directedto a cyclone (12), where collected dust (Z) is removed and sent to theconditioning mill (6). Cleaned air (Y) exits to the atmosphere with alow content of particulates. Optionally a bag house may be used afterthe cyclone (12) in locations with more restrictive particulate emissionlimits.

As an example, applying the process described above to digested poultrymanure would, based on calculations, produce pellets with over 8%nitrogen concentration on a dry mass basis. In comparison, the rawmanure in the calculation has a nitrogen concentration of 4%. Forfurther comparison, simply de watering and drying the same digestatewould result in pellets with no more than 3.5% nitrogen. Although anyammonia recovery would be beneficial, with many feedstocks it should bepossible to add at least 2% to the nitrogen concentration of the pelletson a dry mass basis by way of ammonium salts precipitated from asolution containing ammonia recovered from a liquid fraction of thedigestate.

Referring to FIG. 2, a digester feedstock (alternatively called asubstrate), or combination of feedstocks, (2A) is fed to an anaerobicdigester (21), which is stirred with a mixer (22). The digester (21) canbe arranged in single or multiple stages. Depending on the substrate(s)(2A) the digestate (2B) may contain undigested fibrous material withlignocellulose. In addition to fibers, it may contain undigestedsuspended and colloidal organic matter particles that were not degradedby anaerobic bacteria, inorganic solids, and anaerobic bacteria thatgrow in the digester. Additionally the digestate (2B) is mixed in asludge with water with ammonia, potassium and phosphorous in solution.The particulate solids may also have organic nitrogen and phosphorous.

Digestate (2B) is directed to a solids separator (23). The solidscontent of the digestate sludge may vary, for example, from 3 to 9%depending on the substrate and type of digester (21) and mixing system.The separator (23) is preferably a screw press with openings larger than400 microns but smaller than 1000 microns. The separator (23) produces asolids cake (2D) and a liquid fraction (2E), alternatively called areject, filtrate, centrate or pressate. Other solids separation devicescan be used, such as screens or roller presses. The cake (2D) containsfibers and large particles retained by the press screen and a smallfraction of the small particles in the digestate (2B), including someanaerobic bacteria, that independently would have passed the screenbased on size but became trapped and entrained in the larger particleand fiber matrix. Cake solids content may range, for example, from 20 to35%.

The solids separator (23) and steps treating the cake (2D) mayoptionally be omitted, particularly if there is not a significantconcentration of solids, particularly fiberous solids, in the digestate(2B). A similar cake separation and related step may optionally be addedto the system of FIG. 1 if its digestate contains fibrous solids.

The cake (2D) goes to composting, for example aerobic thermophiliccomposting. FIG. 2 shows an enclosed rotary mechanical drum composter(216) where atmospheric air (2AA) is fed by a blower (218). Thecomposter shown is rotary drum type but other types of in-vesselcomposting processes can be used, such as systems with modified shippingcontainers or plastic agricultural bags. Open windrow composting canalso be used. In the composter, temperature increases as a result ofbacterial and fungal activity. Bacteria decompose simple organiccompounds and fungi process more complex substrates in the compost. Hightemperatures are beneficial for the destruction of pathogenic organismsand undesirable weed seeds that may have survived the intestinal tractof ruminants if the feedstock to the digester includes animal manure,and also survived the anaerobic digestion process. Optionally, other orsupplemental methods of pathogen and/or vector destruction may be used.Decomposition is more rapid in the thermophilic temperature range of135° to 160° Fahrenheit. Foul air (2AB) that contains ammonia, VOCs andsome particulates exit the composter. This air may be treated, forexample in a biofilter. The compost (2AC) may be cured and then used toprepare, or as part of, a plant growing medium or mix.

Synthetic nitrogen fertilizers may be added to the compost (2AC) toincrease its nutrient content and value as a soil enhancing medium.Alternatively, as described in the example of a process shown in FIG. 2,the addition of synthetic nitrogen is not required as nitrogen isrecovered from the screw press (23) filtrate (2E) and incorporated intothe blend as a high nutrient granule or flake (2Y). The process toproduce this granule or flake is described below. A blender (278) blendscompost (2AC) and high nutrient granules or flakes (2Y) to prepare afinal soil medium product (2AD) can be done at the facility where thedigester is located and the organics and nutrient recovery process takesplace. Alternatively, the granules (2Y) and compost (2AC) may be shippedseparately to a blending plant that also has bagging and packagingfacilities. Yet a third option is to sell or use separately the compost(2AC) and the nutrient-rich granules or flakes (2Y). For example, thegranules or flakes (2Y) can be used to enhance compost prepared by otherprocesses. Alternatively, pellets or intermediate products produced bythe system of FIG. 1 may be added to compost on site or in a separatefacility.

The liquid fraction or filtrate (2E) out of the screw press (23) mayhave, for example, 2 to 5% total solids content depending on thedigester feedstock (2A) and the size of the openings in the screw press(23) screen. The dissolved solids in the filtrate (2E) may be, forexample, 1 to 1.5%. The rest of the solids content is suspended solids.Total solids removal in the press (23) varies depending on the digesterfeedstock and the screw press screen size but typical removalefficiencies are about 50% total suspended solids (TSS) and 35% totalsolids (TS). Associated with this separation, a portion of the nutrientsin the digestate remain with the solids fraction out of the press.Typical portion of the nutrients remaining in the cake may be about 25%of N, 50% of P, and 6% of K.

Filtrate (2E) goes to a second step of solids separation. A coagulantsalt (2G) such as ferric chloride or alum is added. A mixer (24)disperses the coagulant in the liquid stream. A dilute polymer (2H) isfed after to flocculate the microflocs formed by coagulant addition. Ashear valve (25) enhances the dispersion of dilute polymer. The liquiddosed with coagulant and polymer enters a rotary screw dewaterer (26).In this dewaterer about 95% of the TSS and 65 to 70% of the TS isretained. Typical rates of N, P and K removal from the liquid are about35% for N, 80% for P, and 8% for P. These nutrients remain in the cake(2I) along with the TSS. The cake has typically 20% to 22% solidscontent. The cake (2I) goes to a blender mill (214), where it iscombined with ammonium sulfate (217) recovered from the filtrate (2J).Introducing ammonium sulfate increases the nitrogen content of theproduct.

In some cases when high rate, short hydraulic retention time anaerobicdigesters are operated, it is important to return bacterial biomass tothe digester. This increases the solids retention time in the digesterand improves process stability. A large fraction of the suspended solidscaptured by the dewaterer and concentrated into cake (2I) is anaerobicbacteria. A portion of the cake (2I1) can be returned to the digester ifneeded or desirable for the operation of the digester (21). This can bedone using a positive displacement pump. Optionally, a portion offiltrate (2E) may be returned to the digester (21). Both of streams (2E)and (2I) are advantageously reduced in liquid content, which helpsincrease the solids content in the digester. Stream (2I) is preferred asa means of solids recycle because of its higher solids content andreduced ammonia content relative to stream (2E).

The blend of cake and ammonium sulfate (2X) is fed to a thermal dryer(215). The dryer depicted in FIG. 2 is a low temperature direct beltdryer. Other dryers can also be used, such as indirect hollow screw,disc, thin film, direct drum, etc. The dryer removes moisture and leavesthe solids. However, depending on the pH of the mixture and the dryertemperature, a portion of the ammonia in the cake/ammonium sulfate blendmay volatilize and escape the dryer along with the evaporated water.Ammonia loss is minimized by reducing the pH below 6, such that most ofthe ammonia exists as ammonium ion and not as unionized ammonia gas thatvolatilizes. The pH reduction to 6 results in less than 5% ammonia losswhile drying at 105 degrees C. Higher drying temperatures such as indirect drum dryers require reducing the pH to 5 to maintain the lossesin the same range. Sulfuric acid (2AL) can be dosed into the ammoniumsulfate line (217), to make a more acidic ammonium sulfate solution.Alternatively, ammonia may be recovered from dryer vapor as shown inFIG. 1.

In the direct low temperature belt dryer shown in FIG. 2, atmosphericair (2AE) is fed with a blower (211). The air is heated in a liquid togas heat exchanger using waste heat as hot water (2AH1) in a closed loopfrom an internal combustion engine running on biogas (2C) or othersource of waste heat. The return heating water (2AH2) goes in a closedloop to the waste heat source. If the available waste heat isinsufficient to meet the requirements of the dryer, a fuel fired airheater (213) is used in addition to the waste heat air heater. The fuel(2AG) can be gaseous or liquid. The hot air (2AF) enters the belt dryer(215). Hot air (2Z) may go to air treatment to remove particulatesand/or to a biofilter or thermal oxidizer depending on local emissionstandards. Hot air (2Z) can also be used as a heat source anywhere elsein the process requiring heat. The granule or flake exiting the dryer(2Y) has a high content of nitrogen and phosphorous. The concentrationsdepend on the N and P content of the feedstock (2A).

The filtrate (2J) out of the dewatering device (26) goes to an ammoniastripping unit (27), optionally called a stripper. The ammonia strippingunit may be contained in an enclosed vessel, for example a rectangularbox, and operates with a low liquid level, for example 1 m of depth orless, usually about 0.6 meters of depth. The volume of the vessel issuch that it provides about 30 to 40 minutes of hydraulic retention timebased on filtrate throughput. The stripper receives subsurface diffusedair (2M) through medium bubble diffusers, and surface crossflow sweepingair (2L). The stripper operates at above ambient pressure, for example50 degrees C. or more, or about 70 degrees C. or more. The stripper isheated by recirculating stripper effluent (20) with a hot watercentrifugal pump or circulator through a liquid/liquid heat exchanger(28). The heat exchanger is part of a hot water loop (2Q1 and 2Q2) andemploys as a heat source waste heat from an internal combustion engineoperating on biogas (2C) or another heat source. The heated return (2P)is directed to the inlet of the stripper (27). The stripper operateswith multiple stages, for example 3 to 5 stages. The stages may bedivided with perforated baffles or by other means such as a weir orpiped connection. Bubble diffusers are placed in each stage. After thelast stage the stripper has an overflow weir that controls the level ofthe water in the stripper. An internal reservoir at the end of thestripper allows the effluent to de-aereate such that it can be pumpedfor recirculation heating or directed as effluent (2N) to a storagetank. Effluent (2N) can be used as dilution water for the digester, ifrequired, or sent for disposal optionally after further treatment. Insome cases mechanical de-aeration devices may be required in thecirculation loop.

The stripper can remove ammonia without adding chemicals for pHincrease. At 70 degrees C., the diffused air drives carbon dioxide outof the liquid. The crossflow air introduced at the surface of the liquidfurther reduces the concentration of carbon dioxide in the headspace ofthe stripper. This enables increased CO2 stripping. The CO2 is in thefiltrate as ammonium bicarbonate which results from the digestionprocess and is in equilibrium with the high CO2 content of the biogas inthe digester headspace, usually 30 to 45%. Stripping raises the pH to9.2 or higher. At this high pH and high temperature, the majority of theammonia becomes unionized ammonia gas in the filtrate and is driven outof solution and into the stripper headspace by the subsurface diffusedair.

The surface crossflow air reduces the ammonia concentration in thestripper headspace at the interface between water and air. This is anequilibrium reaction. The dilution of the headspace facilitates ammoniaremoval due to the higher concentration gradient between the liquid andthe air above it.

The combination of subsurface diffused air and crossflow air now ladenwith ammonia and CO2 (2K) is driven out of the stripper headspace by aslight negative pressure created by an induced draft fan (210),optionally part of a downstream ammonia acid scrubber. As a calculatedexample, a digestate filtrate flow of 170 gpm containing 5000 mg/L ofammonia will require 8,000 scfm of diffused air and 12,000 cfm of lowpressure crossflow air to reach 90% ammonia removal operating at 70degrees C. and pH 9.3. The ammonia concentration in the air outletstream (2K) is 8000 ppm by volume.

Subsurface air is introduced by a blower. In cold climates thesubsurface (2M) air can be heated prior to entering the stripper using agas to liquid heat exchanger (219). This exchanger can be placed isseries with the recirculation heat exchanger (28) such that the incominghot water (2AJ1) is the outlet water (2Q2) of the recirculationexchanger (28). This enables more efficient use of the waste heat. Othersources of heat can also be used. The heat demand for the flow rate inthe example above is approximately 3 MW and a portion of it is used tomake up for the heat of evaporation, as a small fraction of the water islost to evaporation.

The crossflow air uses less energy per unit of flow than the subsurfaceair. The flow rate of the subsurface air is less than half of the flowrate of the cross flow air. For example, the subsurface air may be 15 to45% of the total air flow. The headspace may be restricted to a lowheight or volume, for example 1 m or less. Waste heat, for example froma turbine burning the biogas C, can be used to heat the air or feedliquid. A high temperature in the stripper helps prevent phosphate saltsin the feed liquid from settling as the pH rises. Cooling the effluent(2N) after it exits the stripper allows these salts to be precipitatedin a controlled location such as a storage tank.

The ammonia-laden air (2K) goes to an ammonia acid scrubber (29). Thescrubber uses a counter flow column configuration with air circulatingfrom the bottom up through a packed bed with plastic media to enhancegas/liquid mass transfer surface area. An acid shower with excesssulfuric acid (2V) flows from the top down and reacts with the ammoniagas in the air stream to form ammonium sulfate. Ammonium sulfate isstored in a sump at the bottom of the scrubber column. Ammonium sulfate(2T) is pumped for recirculation and sulfuric acid (2U) is added.Sulfuric acid addition is controlled automatically based on a pH setpoint. Excess sulfuric acid can be added to the recirculation stream toproduce an acidic ammonium sulfate solution to reduce ammoniavolatilization in the dryer. This is an alternative to sulfuric acidinjection (2AL) to the product ammonium sulfate stream. The acidscrubber (29) produces 28 to 30% ammonium sulfate solution when noexcess sulfuric acid is dosed. The ammonium sulfate stream (W) goes to amixer (14) to combine with solids cake (I) and then to the dryer (15).The scrubber exit air with low ammonia concentration (2R) is moved by afan (210) and discharged to the atmosphere (2S). As an alternative tosending 30% ammonium sulfate solution to the dryer, a concentrationsystem (217) can be used that concentrates the solution to 68%. Theconcentrator uses waste heat (2AE1 and 2AE2) and vacuum (2AI) andoperates at about 70 degrees C. In some cases removing moisture from theammonium sulfate solution is cost effective compared to removing thismoisture in the dryer, mostly when dealing with indirect dryers that aremore expensive than direct belt dryers. Alternatively, an ammoniastripping and recovery device described in relation to FIG. 1 can beused.

Any suitable process steps or equipment from FIG. 2 can be used in placeof similar process steps of equipment in FIG. 1. Any suitable processsteps or equipment from FIG. 1 can be used in place of similar processsteps of equipment in FIG. 2. Either system can be used to treat sludgefrom a wastewater treatment plant. Ammonia reduced liquid may bereturned to the wastewater treatment plant.

Example

Waste activated sludge was treated in an anaerobic digester at amunicipal wastewater treatment plant. Sludge from the digester wasdewatered with a centrifuge to produce a centrate. Ammonium bicarbonatewas added to the centrate to reach ammonia and alkalinity concentrationstypical of a high solids anaerobic digester used to treat industrial andagricultural substrates. No other chemicals were added. The modifiedcentrate was treated in an ammonia removal device and process asdescribed above. Concentrations of ammonia and alkalinity were measuredin the centrate feed to the ammonia stripping device and in the liquideffluent removed from the ammonia stripping device. The results of theexperiment are shown in Table 1 below. Process conditions and parametersand device (reactor) volumes during the experiment are given in Table 2below.

TABLE 1 Alkalinity NH3—N % Removal % Removal Sample mg/L as CaCO3 pHmg/L Ammonia-N Alkalinity Feed 23750 8.6 7400 Effluent 3750 9.2 1000 86%84%

TABLE 2 Diffused Crossflow Reactor Reactor Volume Reactor (bubbled)(surface) Volume No Air With Air Influent HRT Temperature Air Air galgal gpm min Deg Celsius scfm scfm 4.56 7.13 0.128 36 67 20 40

U.S. Provisional Patent Application Nos. 61/443,905; filed Feb. 17,2011, and 61/578,703; filed Dec. 21, 2011, are incorporated byreference.

We claim:
 1. An apparatus for treating sludge comprising, (a) one ormore of a mechanical dewatering device and a dryer configured to receivethe sludge and to output a solids fraction and a liquid fraction; (b) anammonia recovery system to receive the liquid fraction and output anacidic ammonium salt solution; (c) a pellet making machine to receivethe solids fraction and the acidic ammonium salt solution and to combinethem into pellets.
 2. The apparatus of claim 1 further comprising an airflow drier to receive and dry the pellets.
 3. The apparatus of claim 1wherein the ammonia recovery system comprises one or more of a steambased ammonia stripper and ii) a vacuum induced ammonia gas extractor.4. The apparatus of claim 3 wherein the ammonia recovery system furthercomprises an ammonium salt solution concentrator.
 5. The apparatus ofclaim 1 further comprising an anaerobic digester to produce the sludge.6. A process of making a fertilizer pellet comprising combining driedsolids extracted from anaerobic digester sludge with an acidified liquidcomprising ammonia previously removed from the anaerobic digestersludge.
 7. The process of claim 6 further comprising drying the pelletby way of a flow of air at 90 degrees Celsius or less.
 8. A processcomprising steps of a) recovering fibers and a filtrate from ananaerobic digester sludge; b) recovering solids and a liquid from thefiltrate; and, c) recovering ammonia from the liquid.
 9. The process ofclaim 8 wherein the fibers are used in a plant growing medium.
 10. Theprocess of claim 9 wherein at least some of the solid or liquids areadded to the plant growing medium.
 11. The process of claim 8 wherein atleast some of the solids or liquids are converted into a granule orflake form.